The chairman of outsourcing firm Capita is stepping down over “spurious” claims his £1m loan to Labour resulted in the group getting government contracts.
Rod Aldridge, one of 12 donors who lent the party almost £14m in total before the last election, said he did “not want this misconception to continue”.
Chancellor Gordon Brown said the political funding system had to be reformed to increase “transparency”.
An influential Commons committee has also launched an inquiry into funding.
The constitutional affairs committee will interview Lord Levy, Tony Blair’s chief fundraiser, on his role in persuading Labour backers to make loans.
Lord Levy will also be questioned by the public administration committee.
Tony Blair has denied suggestions that he nominated wealthy supporters for peerages in return for Labour Party loans.